Methods and apparatus of installing “blown fiber” are well known, wherein one or usually more of a number of optical fibers are “blown” into a previously laid conduit or tube with the assistance of a fluid drag to propel the fiber(s) along, as described in e.g. EP108590. Currently, this method is used by the applicant to install single fibers or more usually, a fiber unit comprising a number of individual fibers. An optical fiber measures mere microns in diameter, and the versions of blown fiber units installed by blowing techniques require little strengthening, so such fiber units are relatively lightweight and pliable. One version of a fiber unit used by the applicant, known as Enhanced Performance Fiber Units (EPFUs) is about 1 to 2 mm in diameter.
Installation of blown fiber is performed using a “blowing head” which essentially comprises a pair of electrically-powered drive wheels between which the fiber unit is driven into a tube coupled to the blowing head. A compressor is connected to the blowing head which directs a supply of compressed air into the tube through which the fiber unit is being driven. Various versions of blowing heads are known, such as those described in EP108590 (supra.), WO98/12588, or WO2006/103424. As noted above, the fiber unit being installed is lightweight and pliable and susceptible to buckling especially as the tube which the fiber unit is to occupy may be more a kilometer long. In such cases, friction within the tube may slow down or even stop the progress of the fiber unit within the tube. Parts of the fiber unit could also get caught within the fiber tube, and the reduction or cessation of movement of the fiber unit into the tube is transmitted back to the blowing head.
To deal with the ill effects of fiber buckle (which may include compromise of fiber integrity in performance or even physical terms), the blowing head covered by WO2006/103424 (supra.) (referred to here as the “current capping” blowing head or method) includes a feature providing that the electrical current supplied to the drive wheels is capped. When the progress of the fiber unit slows or stops, this is transmitted to the drive wheels, which correspondingly slows or stops to avoid putting excessive force on the fiber captured between the drive wheels.
Blown fiber is a significant installation technique in the push for Fiber to the Home (FTTH), wherein an all-optical network is envisaged to supply customers (i.e. private, residential customers in addition to commercial or industrial parties) with an optical connection between the access network direct to the customer's premises or home, or at least a good way thereto. As optical fiber is rolled out deeper into the access network however, capacity and congestion in optical fiber ducts is fast becoming a serious problem. One solution is the use of “blown cables”, which are polymer sheathed optical fiber cable consisting of between 12 to 288 loose tube fibers or ribbon fiber, disposed, in some cases, individual “loose tube” fibers (typically 8 to 12 fibers per loose tube) housed in a polymer outer sheath, and are typically more than 10 mm in diameter. They are typically supplied and dispensed from a cable drum of significant mass mounted on conventional cable drum trailer. Blown cable therefore provide a much higher fiber count than fiber units for the space they occupy in the fiber ducts and are increasingly being deployed in preference to fiber units where space is scarce.
A slightly smaller sized fiber cable called “minicables” (also known as “microcables”) is also deployed. The cable sheathing on such minicables is generally relatively thin to reduce the overall cable diameter, allowing for more minicable to be installed in a limited space. It is important to ensure that the fragile cable elements and optical fibers under the external sheath are handled and installed in a way so that they do not suffer from excessive stress during installation. In the main, minicables are configured in much the same way as fiber cables in being also a polymer sheathed cable, but with a smaller fiber count of between 12 to 96 fibers, measuring about 5 to 7 mm in diameter. The term “cable” and “blown cable” used in this description shall refer to either or both such cables, where the context permits.
“Blown cable” is a method of installing such blown or fiber cables and minicables using techniques and blowing heads much like that used for the more lightweight and flexible blown fiber units. This method is also known as “jetting”. The blowing heads typically include drive wheels for mechanically driving the cable into the tube, and direct a supply of pressurized air from a compressor (typically 10 to 12 bar) into the bore of the conduit to provide the cable with a fluidized bed to help propel it along. However, blowing heads for installing blown cable have to be adapted for blowing the larger (in diameter), heavier and stiffer blown cable and minicables, which are supplied in the form of a cable drum having significant mass on a cable drum trailer.
The apparatus and devices used for installation of minicable comprise components which are larger in size than a standard fiber unit blowing head used for optical fiber units, to accommodate the greater overall dimensions and weight of minicable. An additional problem posed by the size and weight of the cable and the way minicable is supplied on large storage drums, is that while hauling off the minicable from the supply drum, a high level of inertia needs to be initially overcome. The resistance encountered during hauling the blown cable off its drum creates back tension on the cable which pulls against the action of the drive wheels to drive the cable into the waiting tube. Thus when the drive wheels haul the cable off the storage drum, the section of cable leading from the drum to the drive wheels of the blowing head is under considerable tension. This can be contrasted with the experience with blowing lightweight and flexible fibers or fiber units, where there is very little back-resistance or back-tension in the fiber as it is conveyed from the fiber storage pan to the drive wheels of the blowing head.
The problem is exacerbated by the cable coming off the drum being played out in an uneven, jerky manner. This is especially so when the end of a particular cable “layer” on the drum is reached. It is well known that while blowing fiber, especially through long tube routes (which currently could exceed 1 km in length), friction and other causes cause the fiber to suffer significant compression and tensile forces within the tube, causing the fiber to buckle. This may compromise the delicate fiber within the blown cables, as well as result in installation delay and or even for the session to be aborted so that it must be restarted. These issues are also experienced in installing blown cable (which shall in this description include both blown fiber cable and minicable).
Ideally, the cable should be blown into the tube as smoothly and evenly as possible, which is likely to be difficult given the propensity of the cable to be installed in a jerky manner into the tube at one end, and the uneven hauling off of the cable at the other end. To help smooth out the installation process at the tube end, the applicant has developed the current capping system blown fibers wherein the blowing head is capable of sensing an impending buckle—which is manifest in the form of a reduction in speed or cessation of movement of the fiber unit captured between the pair of drive wheels. When the impending buckle is sensed, the motor powering drive wheels reduce or stop the drive force which propels the fiber forward, as its current is capped.
In blown cable, however, the pulling and pushing (and any ancillary vibrational) forces inflicted on the cable at each end of the cable—frictional forces within the tube at one end, and inconsistent cable drum play out from the other—results in spasmodic cable action at the drive wheels of the blowing head, which plays havoc on the installation process. The cable itself undergoes considerable stress as well.
Examples of cable blowing heads are manufactured by Plumettez S.A. of Switzerland (the “MINUET” [trade], where the driving means comprise a pair of motor-driven belts) and CBS Products Ltd of the UK (the “Breeze” cable blowing machine). To protect the cable, the cable blowing heads of the prior art are often configured so that the motors powering the drive wheels stall when the friction between the cable and the tube exceed a pre-set limit. This is of course disruptive, and creates delay in the installation process.
There is therefore a need to address the problem of obtaining a smooth and even supply and conveyance of fiber cable or minicable to the driving mechanism of the blowing machine or head for installation using compressed air, and thence in similar fashion into and through the duct or tube.
An approach to protect the cable from continuing to be driven at a specific speed even when the friction between the cable and tube bore is such that actual cable progress is less than the driving speed, is to establish a preset with electrically powered motor current limit. When the limit is reached, the motor stalls, as would be the case with a hydraulically or air driven system.
One solution developed by the applicants is described in WO 2008119976, wherein the forces required to overcome the cable drum mass and pull the cable off the drum, and the installation force required to insert the cable into the tube with aid of compressed air, are separated. This is achieved by the use of two separate sets of drive wheels, one to pull the cable off the cable drum, and another to push the cable into the tube. The wheels are driven by four motors. The cable is driven through the two sets of drive wheels, and the weight of the cable forms a natural catenary between the sets of wheels. This catenary or cable bend is used to detect when there is a reduction in the rate of installation of the cable into the tube: when the bend or curve exceeds a pre-determined value, there is deemed to be a problem or a potential problem with the rate of installation so that the mechanical force driving the cable into the tube can be adjusted. The rate of catenary bend in the cable is detected using a dancer arm. This method relies on the mechanical properties of the cable: that it is able to form a catenary in the first place (as cables tend to vary considerably in terms of weight and stiffness), and if not the extent to which a bend can be induced or formed in the section of cable between the two sets of wheels. This limits the range of cable that may be used with such a machine, and may produce inconsistent and unexpected results with different cable types. In tests, the apparatus and methods of this invention has also been found to be complex to manufacture and to operate owing to the use of four motors and the arrangement of wheels to enable or induce a catenary in the cable between them.
Another cable installation machine is described in WO 9912066, in which a continuous drive assembly powered by a hydraulic motor is arranged to drive the cable into and through a duct. The speed at which the drive assembly operates relative to the speed of progress of the cable is monitored. In use, a certain amount of push force from the drive assembly is required to overcome obstacles in the form of irregularities, joints and bends present in the duct. When such irregularities are encountered, the speed at which the drive assembly is operating is greater than the speed of the cable, resulting in a certain amount of differential speed between the operating surfaces. Excessive differential speed could however result in cable slip (so that the cable surface rubs against the drive mechanism instead of being carried along with it) which in turn could damage the cable jacket or the cable itself. The apparatus in this document is set up so that if the speed of the drive assembly exceeds that of the cable by a predetermined amount, e.g. 15%, the drive assembly and their motors are shut down. In other words, the system shuts down only in the existence of what is deemed to be potentially catastrophic conditions, upon detection of cable slip of a magnitude which is deemed to be of an unacceptable.
Such a system suffers from various shortcomings. First, a figure has to be selected as the predetermined difference value: this is one probably based on historic data, trial and error, or worse still, a completely random number. As is well known, each installation operates under potentially vastly different conditions (length of installation, weather conditions, the type of cable in question and parameters such as weight, stiffness and diameter/subduct bore ratio, how convoluted and jointed the cable pathway is, how crowded the cable route is already, and so on)—any predetermined figure cannot be work optimally or near-optimally in all such cases. Furthermore, the “correct” predetermined value would vary at different stages during the installation process owing to changes within the duct in the cable pathway, in the level of friction existing between the cable and the duct, in the extent of the air cushion around the cable, especially at the far end of the duct, and so on. Based on this unreliable predetermined figure, the system is set up to take the drastic step of shutting down completely whenever the predetermined value is observed. If there was no risk of damage to the cable, this of course results in significant waste in manpower time and effort. At the same time, the cable remains at risk of damage if the predetermined value is set too low in the particular instance, e.g. where the irregularities within the duct fail to slow cable progress sufficiently to exceed the set predetermined value.
It would be desirable to address the above problems to enable smooth and consistent installation of blown cable.